"The joy of being in the water, suspended in silence, at that depth, is incredible," said William Trubridge, who has won more than a dozen world championships.

In a CNN iReport, Trubridge described what it's like. "There's a lot of physiological changes that happen," he said.

"The main thing which isn't experienced in any other sport is what's called the 'blood shift,' which is where all the blood is squeezed in from your extremities, and at the same time your lungs are becoming crushed by the weight of the water column above you. So you get blood pooling in the lungs."

"It's a mental sport as much as it is a physical one. One of the beautiful aspects of it is that it forces you to be in the moment. It's almost impossible to be in the water and at the same time contemplating problems. As soon as you get in the water, that all dissolves and you're just there."

In 2006, he lost consciousness before getting to the surface. He also lost his sense of taste, which has never returned.

There are no clear figures on how frequently deaths and injuries occur. "The statistics are a bit murky," Outside magazine reported. "Some deaths go unreported, and the numbers that are kept include people who freedive as part of other activities, like spearfishing. But one estimate of worldwide freediving-related fatalities revealed a nearly threefold increase, from 21 deaths in 2005 to 60 in 2008."

Photographer Logan Mock-Bunting, who captured stunning images of a 2012 competition, said that despite dangers, serious freedivers are hardly reckless. They're followed by safety divers and can be brought back to the surface, where a medical team is on hand.

"Ultimately, it's the professionalism and constant training of these staff that give the athletes the utmost confidence that they can push their limits and work at top levels," Mock-Bunting said. "The system around them is so strong."

"You begin to imagine the efficiency of function a fish or dolphin must realize with their superior hydrodynamic designs," the journal says, "and wonder why you took so long to experience this natural euphoria."